Typical engine exhaust is a heterogeneous mixture which contains, among other constituents, gaseous emissions such as carbon monoxide (CO), unburned or partially burned hydrocarbons or oxygenates thereof (HC) and nitrogen oxides (NOx). Often, catalyst compositions and substrates on which the compositions are disposed are provided in engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For example, three-way (TWC) catalysts are known to be suitably employed on stationary rich-burn engines to reduce the emissions of nitrogen oxides (NOx), hydrocarbons (HC), and carbon monoxide (CO). Because these engines operate under rich conditions in which the air-to-fuel ratio has an amount of fuel that is greater than stoichiometric (i.e. the air-to-fuel ratio is below the stoichiometric ratio), a significant portion of the engine out NOx is converted to ammonia (NH3) over the three-way catalysts and is consequently emitted as a secondary emission. For a typical rich-burn engine that is equipped with a TWC catalyst, the tailpipe NH3 can be around 400 ppm.
A proven NOx abatement technology applied to stationary sources with lean exhaust conditions is Selective Catalytic Reduction (SCR). In this process, NOx is reduced with a reductant, such as ammonia (NH3), to nitrogen (N2) over a catalyst typically composed of base metals. SCR provides efficient conversions of NOx as long as the exhaust temperature is within the active temperature range of the catalyst. Reduction of NOx species to N2 using NH3 is of interest for meeting NOx emission targets in lean burn engines. A consequence of using NH3 as a reductant is that under conditions of incomplete conversion or exhaust temperature upswings, NH3 can slip from the exhaust of the vehicle. To avoid slippage of NH3, a sub-stoichiometric quantity of NH3 can be injected into the exhaust stream, but there will be decreased NOx conversion. Alternatively, the NH3 can be overdosed into the system to increase NOx conversion rate, but the exhaust then needs to be further treated to remove excess or slipped NH3. Even at a substoichiometric dosage of NH3, an increase in exhaust temperature may release ammonia stored on the NOx abatement catalyst, giving an NH3 slip. Conventional precious-metal based oxidation catalysts, such as platinum supported on alumina, can be very efficient at NH3 removal above 225° C., but they produce considerable N2O and NOx as undesired side products instead of the desired N2 product. Generally, the use of diesel engine catalysts for gasoline engines, stationary diesel or natural gas engines, results in the generation of NH3 emissions since the catalyst is very active.
Two-staged systems which employ a staged NOx treatment-NH3 treatment configuration are known in the industry. The catalysts employed by these systems, however, fail to achieve high selectivity of NH3 to N2 at temperatures above 400° C., where excessive oxidation of NH3 to NOx can lead to the system exceeding NOx regulations. Additionally, the prior art systems typically relate to lean-burn engines that are operated above the stoichiometric ratio. Given this lean-burn condition, most of the prior art systems employ a catalyst for the selective catalyst reduction (SCR) of NOx. SCR of NOx by nitrogenous compounds, such as ammonia or urea, has developed for numerous lean-burn applications including for treating industrial stationary applications, thermal power plants, gas turbines, coal-fired power plants, plant and refinery heaters and boilers in the chemical processing industry, furnaces, coke ovens, municipal waste plants and incinerators, and a number of vehicular (mobile) applications, e.g., for treating diesel exhaust gas. Similar to the known two-staged systems, however, known SCR catalysts and systems fail to achieve high selectivity of NH3 to N2 at temperatures above 400° C., where excessive oxidation of NH3 to NOx can lead to the system exceeding NOx regulations.
Currently, there is no regulation on NH3 emissions for exhaust from combustion systems because there is no technology commercially available to reduce the NH3 emissions in such systems, while also meeting the stringent emission regulations for NOx, HC, and CO. It would thus be useful to identify catalysts, and systems which employ the catalysts, which can minimize NH3 emissions, for example below 10 ppm, while maintaining NOx, HC, and CO emissions below existing regulations. Accordingly, a catalyst and system which reduces the emissions of nitrogen oxides (NOx), hydrocarbons (HC), and carbon monoxide (CO) from the exhaust of a rich-burn engine operating at high temperatures, and which provides an NH3 oxidation functionality to reduce the amount of NH3 in the effluent, remains highly desirable.